Encoder Cable Length

Have a project where an incremental encoder (push/pull 24V d.c. output) will be 300 metres away from a Siemens S7-226 PLC. I am worrying that I will loose the signal in the cable, am I worrying about nothing? Have done some calculations and I reckon that at 55mA I would only loose 1 Volt, but long cable runs and noise always worry me. Frequency will be 200Hz maximum. My other option is to choose an encoder with a differential output and then before the PLC, invert one of the signals and combine them so that any noise cancels out, but I haven't been able to find a simple device to do this (probably me being thick). Either way I would obviously use screened cable, and with twisted pairs for the differential system.

I could get an encoder with a fibre output but the cable has to reeled in and out a lot so breakage would worry me. One end of the system is underground so wireless is out.

What's your thoughts?

Bryan

Too long a run IMO.

I have extended the distance for an encoder once years ago, but it was about 200' not 300m. We clipped the pin on the servo drive that supplied the +24vdc to the encoder and wired in a larger 24v power supply, then made sure we tied the servo and power supply 24 vdc commons (24-) together. It worked like a champ, but again, we wer eonly going about 200'. Any chance you could use a DeviceNet or Profibus encoder?

BryanG said:

the cable has to reeled in and out a lot so breakage would worry me.

Bryan

Click to expand...

Does this mean the signal has to go through a slip ring also? If this is true, your chance of success is very small indeed.

Is there any posiblity of putting a small plc right at the encoder and using some sort of comm. between them? Or maybee just a few discreet outputs?

My impression tells me its OK.

200 Hz is slow.
24 V is a lot.

I would be more concern if you use to big a gage.

BUT ... this is such a easy thing to test.

Go an purchase a box of Cat5E cable ... gage 24.

Cost you less than $100.

Its 1000 feet and you got both end of the cable accessible... Test it without taking any cable out of the box.

PS: You could also do the maths... resistance, capacitance, voltage .. RC curve ... threshold... Voila!

Cheaper to test.

I'm sure you'd get some trustworthy information at http://www.beiied.com/incremental_encoders.html

Why not connect the 300m at your office and use a simple digital counter. Connect the encoder (supply, and signal) and see if the signal comes through correctly. Do 1 rev and see if the pulses are correct

Thanks All

After some serious Googling I found these guys who give a 1Km distance for a differential ouput encoder but their differential to single ended converter wasn't suitable. I then found this company (looks similar to beiied.com thanks Doug) who do encoders and a converter that does RS422 to HTL (24V push/pull). That meant some research on RS422 and found that the 4000ft (1200Metres) at 100KHz is do-able. So 300 metres at only 200Hz over RS422 should be OK.

Yes milldron the signal will have to go through a slip ring but I think if I do the RS422 to single ended conversion before the slip ring (in the middle of the cable reel) then I will have two strong 24V quadrature signals over only a couple of metres through the slip ring and back to the PLC.

So in theory I should be more than OK but like Pierre says in the end you just have to try it. I will report back with the results.

Thanks

Bryan